run by adjusting the chamber ’s or blade holder’s position set-
ting or loading pressure.
After all of the measurements are taken and indicated on
the digital display, the image is saved to a file that will later be
compiled in the report. This process is then repeated for each
sample. When all of the samples have been measured in the
microscope, the data is compiled in a report that will contain
commentary about the results of the analysis.
Comments will explain what was measured, what it means,
and any recommendations to help improve the doctoring
efficiency.
ANALYSIS & FINDINGS
So, what does the analysis uncover? Many times the
analysis reveals that the doctor blade was being run at a
contact angle that is too flat (Figure 2) by generally accepted
standards. What is too flat? Accepted contact angles for flexo-
graphic printing will fall into the range of 25 to 42 degrees with
a desirable angle being near 30 degrees (Figure 3). Less than
25 degrees is considered to be to flat for a flexo doctor blade.
Flat angles will not provide proper doctoring or clean up of
the anilox roll and will leave some surface ink on the anilox
causing a shiny surface appearance. An operator may see
the shiny anilox roll and apply more force to the system to try
to clean up the roll (remember the multiple angles mentioned
earlier). This will work initially as the contact angle changes,
but success will be short lived. Too much force applied to the
doctor blade, will quickly wear in a big footprint, potentially
causing dot gain and inconsistent print.
A flat blade angle could also trap dried ink particles, blade
material, or other debris potentially causing the infamous
anilox roll score lines. Conversely, a contact angle greater
than 42 degrees is considered to be too sharp. An angle that
is too sharp can cause the blade to chatter, resulting in lines
across the web or other print defects and vibration.
To illustrate the effects of flat contact angles, look at the two
blade samples shown in Figure 2 and Figure 3. Notice that the
flat contact angle blade shown in Figure 2 has very little wear
(0.001-in.) , but a very large footprint (0.027-in.). In contrast, the
good contact angle blade shown in Figure 3 has 24 times more
blade wear (0.024-in.), but a footprint that is less than half the size
(0.010-in.) of the flat contact angle blade. The flat angle blade
had to be changed prematurely due to poor print quality. If you
look closely, you can still see some rounding on the top of the
blade from the initial blade tip because the flat angle is wear-
ing “up” from the bottom of the blade. The good angle blade is
wearing “back” from the edge of the blade, as it is designed to
do and provided good blade life with consistent print quality.
Containment blades found in chambered doctor blade
systems are the exception to the above angles. Containment
blades should generally run flatter than the doctor blade.
The range of angles for the containment blade is generally
accepted to be between 20 and 25 degrees. The containment
blade is run flatter and is typically a flexible plastic or thinner
metal blade to prevent debris trapping and back doctoring.
Ideally, debris should pass easily under the containment
blade and then be absorbed in the ink to ultimately be cap-
tured by a filter/magnet in the ink circulation system. If your
chamber is not set up to run the containment blade flatter
than the doctor blade, some chamber designs will allow you
to use a slightly wider containment blade, which will automati-
cally flatten out the contact angle. Most chamber designs
will permit a containment blade that is 1/16-in. to 1/8-in. wider
than the doctor blade.
Other than contact angles, a blade analysis will reveal any
misalignment issues (Figure 4) that are present in the setup.
Proper blade to anilox alignment is very important when using
a chambered inker setup. Both blades should contact the anilox
roll at the same time and evenly across the face of the anilox roll.
If there is any misalignment in any axis, extra pressure will need
to be applied to get the blades to seat properly and provide a
Figure 3: Accepted contact angles for
flexographic printing will fall into the range of
25 to 42 degrees, with a desirable angle being
near 30 degrees.
Figure 2: Many times, analysis reveals that
the doctor blade was being run at a contact
angle that is too flat. - -less than 25 degrees.
Flat angles will not provide proper doctoring
or clean up of the anilox roll and will leave
some surface ink on the anilox causing a
shiny surface appearance.
Figure 4: Proper blade to anilox alignment
is very important when using a chambered
inker setup. Both blades should contact the
anilox roll at the same time and evenly across
the face of the anilox roll. If there is any
misalignment in any axis, extra pressure will
need to be applied to get the blades to seat
properly and provide a good wipe.
76
FLEXO
June
2013 www.flexography.org